docs: def calculate_z2pt5_ngaw2(vs30):
      '''
      Reads an array of vs30 values (in m/s) and
      returns the depth to the 2.5 km/s velocity horizon (in km)
      Ref: Campbell, K.W. & Bozorgnia, Y., 2014.
      'NGA-West2 ground motion model for the average horizontal components of
      PGA, PGV, and 5pct damped linear acceleration response spectra.'
      Earthquake Spectra, 30(3), pp.1087–1114.
  
      :param vs30: the shear wave velocity (in m/s) at a depth of 30 m
      '''
      c1 = 7.089
      c2 = -1.144
      z2pt5 = numpy.exp(c1 + numpy.log(vs30) * c2)
      return z2pt5